1. Technical Field
The present invention related to a development device and an image forming apparatus, such as a copier, a facsimile, and a printer, incorporating the development device, and more particularly, to a development device using two-component developer including magnetic carrier and toner.
2. Description of the related art
In known development devices using two-component developer, fresh developer is supplied to the development device in order to maintain a certain toner concentration while the added developer is discharged from the development device. In this type of development device, old magnetic carrier of the developer can be replaced with fresh carrier little by little by such continuous supply and discharge of the developer. Accordingly, deterioration of the magnetic carrier in the development device is retarded, which can decrease the frequency with which the developer must be replaced.
As the development device to which the developer is supplied and from which the developer is discharged, typically a suitable amount of the developer is retained by forming a discharge opening within a screw transport path positioned close to a developer bearer in the development device, where the condition of the level of the developer is relatively stable and the height of the level of the developer fluctuates in accordance with the increase and decrease in the amount of the developer (as, for example, in JP-H08-022190-A).
By contrast, there are development devices in which an agitation container to agitate the developer is provided separately from the part of the device that actually develops the image (a “development portion”). The developer that has been sufficiently agitated in the agitation container is circulated between the agitation container and the development portion. This system has the advantage that, since the agitation container is separated from the development portion, the development portion can be minimized, thus making good use of the limited space near a photoreceptor (image carrier). In addition, the developer is agitated in accordance with the condition of the developer, and therefore, developer of a toner concentration and charging amount adjusted suitably in the agitation container can be supplied to the development portion. However, a drawback of this system is that, when the amount of developer conveyed from the agitation container to the development portion fluctuates, the amount of the developer in the development portion fluctuates, which may easily adversely affect the development process. Accordingly, a known way to convey a constant amount of the developer from the agitation container to the development portion is a pneumatic system, such as an air pump.
In development devices through which the developer is circulated between the agitation container and the development portion, it is desirable that, when the developer is supplied and discharged, the supplied fresh developer is agitated and mixed with the used developer conveyed from the development portion and then the mixed developer in the agitation container is conveyed to the development portion. However, with a constant amount of the developer being conveyed from the agitation container to the development portion, the amount of the developer in the development portion does not change even as the amount of the developer in the agitation container increases with the supply of fresh developer. In addition, the increase in the amount of developer in the agitation container may cause the developer to overflow.